Radio Telescopes to Keep Sharp Eye on Mars Lander

As NASA's
Phoenix Mars Lander descends through the Red Planet's
atmosphere toward its landing on May 25, its progress will be
scrutinized by radio telescopes from the National Radio
Astronomy Observatory (NRAO). At NRAO control rooms in Green
Bank, West Virginia, and Socorro, New Mexico, scientists,
engineers and technicians will be tracking the faint signal
from the lander, 171 million miles from Earth.

Robert C. Byrd Green Bank Telescope
CREDIT: NRAO/AUI/NSF

To make a safe landing, Phoenix must make a risky descent,
slowing down from nearly 13,000 mph at the top of the Martian
atmosphere to only 5 mph in the final seconds before touchdown.
NASA officials point out that fewer than half of all Mars
landing missions have been successful, but the scientific
rewards of success are worth the risk.

Major events in the spacecraft's atmospheric entry, descent
and landing will be marked by changes in the Doppler Shift
in the
frequency of the vehicle's radio signal.
Doppler Shift
is the change in frequency caused by relative motion between
the transmitter and receiver.

At Green Bank, NRAO and NASA personnel will use the giant
Robert C. Byrd Green Bank Telescope (GBT) to follow the
Doppler changes and verify that the descent is going
as planned. The radio signal from Phoenix is designed to
be received by other spacecraft in Mars orbit, then
relayed to Earth. However, the GBT, a dish antenna with
more than two acres of collecting surface and highly-sensitive
receivers, can directly receive the transmissions from
Phoenix.

"We'll see the frequency change as Phoenix slows down in
the Martian atmosphere, then there will be a big change when
the parachute deploys," said NRAO astronomer Frank Ghigo.
When the spacecraft's rocket thrusters slow it down for its
final, gentle touchdown, its radio frequency will stabilize,
Ghigo said.

"We'll have confirmation of these major events through our
direct reception several seconds earlier than the
controllers at NASA's Jet Propulsion Laboratory will get
the relayed information," Ghigo added.

In Socorro, scientists will collect signals from Phoenix
with antennas of the continent-wide
Very Long Baseline
Array (VLBA), which produces the sharpest images of any
astronomical instrument in existence. They will use the
VLBA's ability to mark the position of objects in the sky
with pinpoint precision to reconstruct the craft's location
relative to other spacecraft at Mars to within about 100 feet,
despite its great distance from Earth.

The VLBA observations will demonstrate NRAO's capability to
provide extremely precise measurements of spacecraft
positions. This capability may be used to improve the
navigational accuracy of future interplanetary missions.

NRAO telescopes have contributed to the success of
several previous space missions.

Very Long Baseline Array
CREDIT: NRAO/AUI/NSF

In 1989, the Very Large Array (VLA) received signals from the
Voyager 2 spacecraft as it flew by the distant planet Neptune.
The combined collecting area of the 27 VLA antennas and their
sensitive receivers made possible a higher data-transmission
rate from the spacecraft, thus enabling scientists to obtain
more images of Neptune, its rings, and its moons.

In 1995, the VLA captured signals from the Galileo spaccraft's
probe as the probe dived into the giant planet Jupiter's
atmosphere. Like Phoenix, the Galileo probe was designed to
send its information to the main spacecraft, which would then
relay the signal to Earth. However, the VLA's direct reception
of the probe's signal measured the Doppler shift in the signal's
frequency and made measurements of Jovian wind speeds 10 times
more accurate than they otherwise would have been.

In 2005, the GBT and the VLBA snagged the signal from the Huygens
probe as it descended into the atmosphere of Saturn's moon Titan.
The Doppler measurements of wind speeds made by NRAO and other
radio telescopes
provided the only wind data from the mission,
because of a malfunction in communication between Huygens and
its "mother ship" Cassini.